Method of making heating elements



Feb. 21, 1956 A. J. HUCK METHOD OF MAKING HEATING ELEMENTS Filed June 23, 1952 United States Patent METHOD OF MAKING HEATING ELEMENTS Alfred J. Huck, St. Louis, Mo., assignor to Knapp-Monarch Company, St. Louis, Mo., a corporation of Delaware Application June 23, 1952, Serial No. 295,074 4 Claims. (Cl. 29-1555) This invention relates to a method of making heating elements wherein the heating element consists of a metallic tube with a coiled resistance element centered therein and surrounded with insulating material which is initially in plastic form and extruded with the resistance element to fill it and enclose it.

One object of the invention is to provide a method of expanding the plastic material so that it fits the interior of the tube comprising the steps of plugging the ends of the tube and pushing the plugs inwardly to contract the length of the body of insulating material and thereby expand its diameter to fit the interior of the tube.

Another object is to provide a method which uses insulating plugs for the ends of the tube through which the lead wires to the resistance element extend as the means for accomplishing the expanding action on the body of insulating material, the plugs being thereafter left in position to serve their purpose as insulators for the lead wires.

Still another object is to provide a method of expanding the plastic insulating material into surface irregularities on the interior of the tube, thereby preventing subsequent axial movement of the insulating body relative to the tube.

A further object is to reduce the insulating material to a ceramic by firing the same after it has been expanded as above referred to.

With these and other objects in view, my invention consists in the disclosed steps of my method of making heating elements, whereby the objects contemplated are attained as hereinafter more fully set forth, pointed out in my claims and illustrated in the accompanying drawings, wherein:

Figure l is a sectional view through a hopper of plastic insulating material showing it being extruded along with the coiled resistance wire to form an extruded heating unit of the type shown in Figure 2, Figure 2 being a side elevation of the heating unit.

Figure 3 is an enlarged sectional view through a finned metallic tube with the heating unit located therein and insulating plugs in the ends of the tube to show one step in the method.

Figure 4 is a similar sectional view showing another step wherein the plugs are pressed inwardly to a final position.

Figure 5 is a plan view of Figure 4 on the section line 5-5 thereof; and

Figure 6 is a sectional view on the line 6-6 of Figure 3.

On the accompanying drawing, I have used the reference numeral 10 to indicate a hopper for plastic electrical insulating heat conducting material in plastic form to be discharged by pressure therefrom through a nozzle 12. A guide tube is located within the hopper 10 and is provided for a coiled resistance element 16 having a lead wire 18 on each of its ends.

As pressure is exerted on the insulating material 14, by suitable means (not shown herein but disclosed in Smith Patents 1,951,176 of March 13, 1934, and 1,987,915

2,735,162 Patented Feb. 21, 1956 of January 15, 1935) the material 14 is extruded as at 20 to form a body of insulating material and it carries with it the coil 16 so that the coil is imbedded in the body as shown in Figure 2, the body being of suitable length for the intended subsequent use of the heating unit consisting of the resistance element and the body 20.

The heating unit 16-20 is slid endwise into a tube 22 shown in Figure 3, which tube is preferably of metal and may be of extruded formation preferably provided with a helical fin 24 therearound for greater efficiency in the use of the completed heating element as a heater for air circulated past the fins and the tube. The outer diameter of the. body 20 is such in relation to the inner diameter of the tube 22 that the body may be slid readily into the tube after about 5 or 10 minutes of drying so that the body can be handled without fracture.

Plugs 26 of insulating material such as porcelain or the like are provided having flanges 23 and bores 30, the bores being provided to receive the leads 18 of the resistance element 16. The insulating body 20 is slightly shorter than the length of the tube 22 as shown so that the plugs 26 can be entered a short distance into the ends of the tube and contact with the ends of the body 20.

The next step in the method is to apply pressure against the plugs 26 in the direction of the arrows A and B in Figure 3 to drive the plugs to the final position shown in Figure 4 with the flanges 28 contacting the ends of the tube 22, which results in contracting the length and expanding the diameter of the insulating body 20 so that it will fit the inner surface of the tube 22 and any irregularities thereof. Preferably irregularities in the form of a helical groove 25 is provided in the tube resulting from the extrusion process for forming the tube with the helical fin 24 thereon as shown in the central portion of Figure 3. Comparing Figure 4 with Figure 3, it will be noted that the space left around the outside of the body 20 in Figure 3 has been filled up in Figure 4 and the body 20 has been expanded into the spiral groove 25.

The plugs 26 may now be retained in position by means of metallic washers 32 placed against their ends and retained in position by the end fins 24a bent over the edges of the washer 32 as shown in Figures 4 and 5.

My method as disclosed for making a heating element is suitable for plastic insulating material such as clay, zircon and water in proper proportions to provide an extrudable plastic material to imbed the resistance element 16 and substantially fill the coil as Well as the space between the exterior of the coil and the interior of the tube 22. After the parts are assembled to the position of Figure 4, the entire heating element may be cured, first by a drying operation at about 200 or 300 F. to drive out the water slowly and second by a firing operation at high temperatures from about 900 to 1400 F., this latter operation causing the insulating material to become ceramic or vitreous-like in nature for good heat transfer and low moisture absorption under conditions of subsequent use.

The plastic insulating body 20, if left in the condition shown in Figure 3 before curing would be quite loose in the tube 22 because of shrinkage; whereas by proportioning the parts as shown in Figure 3 and then pressing the plugs 26 inwardly as in Figure 4, the insulating body is expanded to fit the interior of the tube. This eliminates the air gap between the insulating body and the tube shown in Figure 3 which makes a heating element construction which is comparatively inefficient, the extent of inefliciency depending directly on the amount of the gap since air does not transmit heat as well as would result when the insulating body is in direct contact with the tube.

The 5 or 10 minutes drying time mentioned above loses some of the moisture and most of the shrinkage has taken place during this period. The plugs .26 are loose enough to permit air displaced by the body 20 to be expelled from the tube as the plugs are moved from the position of Figure 3 to the position of Figure 4, the plugs serving as pistons to compress the insulating body from each end while it is still slightly plastic, thus causing the diameter of the extrusion to increase inside the tubing and make good contact with the inner surface thereof and to fill any irregularities such as indicated at 25. I have thus provided a method of swelling the semi-dried insulating body 26 to provide good thermal contact between it and the interior of the tube 22.

The plugs 26 are of insulating material so as to serve as electric insulators for the leads 18 at the ends of the resistance element. They also serve as retainers when the washers 32 are placed against them and the end fins 24a are bent over as described.

,A further advantage of compressing the insulating body 20 lengthwise so as to expand its diameter is to prevent its longitudinal sliding in the tube 22 during shipment and use, which can result it the body is not so expanded. This expansion forces the material of the insulating body into the irregularity 25 of the inner surface of the tube so that a very efiective mechanical lock against internal movement of the insulating body is efli'ected in relation to the finned tubing.

Some changes may be made in the construction and arrangement of the parts of my method of making heating elements without departing from the real spirit and purpose of my invention, and it is my intention to cover by my claims any modified forms of structure or use of mechanical equivalents which may be reasonably included within their scope.

I claim as my invention:

-1. A method of making electric heating elements of the kind specified comprising the steps of first providing a metallic tube positively formed with a predetermined axially-irregular inner surface, said tube containing an axially-disposed coiled resistance element and being filled with an elongated body of electrical-insulating, heatconducting, ceramic-forming material in plastic form, plugging each end of the tube with an electrical-insulating plug which in an outer position prevents endwise movement of the body of plastic insulating material and through which the terminal ends of the resistance element extend, subjecting the plugs to pressure for moving them axially of the tube to inward positions therein, the body of plastic insulating material being thereby expanded in diameter by the plugs so as to substantially fill the tube between them and tightly fit the axially-irregular inner surface of the tube, fixing the plugs in their inward positions in the ends of the tube element, and heating the heating element to reduce said body of plastic material to a ceramic.

2. A method of making electric heating elements of the kind specified comprising the steps of first providing a metallic tube positively formed with a predetermined axially-irregular inner surface, said tube containing an axially-disposed coiled resistance element and being filled with an elongated body of electrical-insulating, heatconducting, ceramic-forming material in plastic form, plugging each end of the tube with a flanged electricalinsulating plug which in an outer position prevents endwise movement of the body of plastic insulating material and through which the terminal ends of the resistance element extend, subjecting the plugs .to pressure for moving them axially of the tube to inward positions therein with the flanges of said electrical-insulating plugs disposed against the ends of the tube, the body of plastic insulating material being thereby expanded in diameter by the plugs so as to substantially fill the tube between them and tightly fit the axially-irregular inner surface of the tube, fixing the plugs in their inward positions in the ends of the tube element, and heating the heating element to reduce said body of plastic material to a ceramic.

3. A method of making electric heating elements of the kind specified comprising the steps of first providing a finned metallic tube positively formed with a predetermined axially-irregular inner surface, said tube containing an axially-disposed coiled resistance element and being filled with an elongated body of electrical-insulating, heat-conducting, ceramic-forming material in plastic form, plugging each end of the tube with an electricalinsulating plug which in an outer position prevents endwise movement of the body of plastic insulating material and through which the terminal ends of the resistance element extend, subjecting the plugs to pressure for moving them axially of the tube to inward positions therein, the body of plastic insulating material being thereby expanded in diameter by the plugs so as to substantially fill the tube between them and tightly fit the axially-irregular inner surface of the tube, fixing the plugs in their inward positions in the ends of the tube element, and heating the heating element to reduce said body of plastic material to a ceramic.

4. A method of making electric heating elements of the kind specified comprising the steps of first providing a metallic tube positively formed with a helical groove along its inner surface, said tube containing an axiallydisposed coiled resistance element and being filled with an elongated body of electrical-insulating, heat-conducting, ceramic-forming material in plastic form, plugging each end of the tube with an electrical-insulating plug which in an outer position prevents endwise movement of the body of plastic insulating material, but which allows the escape of air from the tube, and through which the terminal ends of the resistance element extend, subjecting the plugs to pressure for moving them axially of the tube to inward positions therein, the body of plastic insulating material being thereby expanded in diameter by the plugs so as to substantially till the tube between them and tightly fit the helical groove of the tube, fixing the plugs in their inward positions in the ends of the tube element, and heating the heating element to reduce said body of plastic material to a ceramic.

References Cited in the file of this patent UNITED STATES PATENTS 1,987,915 Smith Ian. 15, 1935 1,997,844 Wiegand Apr. 16, 1935 2,134,752 Ehlers Nov. 1, 1938 2,290,107 Luks July 14, 1942 2,363,130 Horsfeld Nov. 21, 1944 2,375,058 Wiegand May 1, 1945 2,527,890 Pouchnik et a1 Oct. 31, 1950 2,632,833 Huck et al Mar. 24, 1953 FOREIGN PATENTS 488,299 Great Britain July 5, 1938 

